In horror movies, something seemingly innocuous always happens before the terror. Maybe a phone rings and no one is on the other end. Maybe a window is found open, even though someone just closed it.

Maybe there’s a knock on the door.

The knock on Sarah Andrade’s front door on a recent afternoon was not exactly terrifying. But the uncanny timing of it speaks to fears in Northern Virginia, and in particular Arlington County, where Andrade lives and where Amazon plans to build one of two new headquarters.

A day after Amazon announced it was splitting its investment between Long Island City in New York and Crystal City, Andrade logged on to the social networking site Nextdoor and started reading through her neighbors’ comments about the company’s impending arrival.

Some described it as a “win” for the area. Others questioned how the county planned to preserve affordable housing. One person warned, “You will see a house buying experience like never before. Always cash and always bidding up the price. Rent will go out of most people’s reach and nothing will ever look the same.”

Somewhere in the middle of reading all those comments, Andrade heard the knock.

When she opened the door, a real estate agent from Keller Williams wanted to know whether her family or anyone in her neighborhood was interested in selling their houses.

Arlington has long been a desirable place to live, with many homes drawing bidding wars and selling for more than $1 million even before Amazon promised to bring 25,000 high-salary jobs to the region. The question raising fears now is how much of an impact the company will have on housing across the area and how it will change the county’s character.

In recent days, many residents have expressed concerns that Arlington will become Seattle (sorry, Seattle). In the city where Amazon is headquartered, homelessness and inequality have risen as homes have become less affordable. Already, according to an article by my colleagues Tracy Jan and Kathy Orton, people familiar with the Washington region’s housing market are predicting Amazon will exacerbate inequality, making Arlington less racially and economically diverse.

That should terrify us.

Beyond the county’s obvious selling points for Amazon — including its highly educated workforce and ease of access to public transportation — one of Arlington’s major strengths is its diversity. It is one of the main reasons many homeowners chose to move to the county.It was one of the reasons my husband and I did.

We have lived in the county for nearly a dozen years and have been homeowners here for seven. We picked Arlington as the place where we wanted to raise our children, drawn by the possibility that we could send them to a goodpublic school and not predict the race or ethnicity of their best friend or what that child’s parents might do for a living.

We’re not alone in that. In our ultracompetitive society, it may seem a given that all parents would send their children to the closest school with the best test scores. But that is not always the case in Arlington. I have had discussions with several parents who have spent time considering whether it’s better to send their children to a closer school with higher scores or to have them travel a bit farther to attend a school with lower scores but more diversity.

Andrade, who received that knock on her door, has a 6-year-old daughter in first grade and said there are about 60 languages spoken in her school.

“I think that would be a true crime to lose that diversity and that exposure to learning compassion and understanding of one another’s cultures and beliefs,” she said.

Andrade said her family is “emotionally and financially” invested in the county. She moved here in 1998, and she and her husband, John, bought their current home in 2012. He also has plans to open Meridian Pint, a restaurant with a location in the District, in the Dominion Hills shopping area in April.

“Overall, I am in the camp that I think it will be a good thing for the community,” Andrade said of Amazon’s decision. “It can ultimately be something that will strengthen Arlingtonians.”

As for that knock on the door, she and her husband believe it won’t be the last. And they are probably right.

Local real estate agents are already seeing the influence of Amazon, which was founded by Jeffrey P. Bezos, who also owns The Washington Post. The new headquarters will be located in an area that has been dubbed “National Landing” and includes parts of Crystal City and Pentagon City in Arlington and Potomac Yard in Alexandria.

“When word hit that Amazon was potentially looking to move its headquarters into Virginia, everyone was already talking about how this would mean a lot of business for Realtors — and they were right,” said real estate agent Laurie Jimenez, who helped my family when we were looking to buy a new home in Arlington.

She said she expects that the housing market will not only see a rise in real estate prices, but also an increase in demand for rentals because of the demographic of the talent Amazon is seeking: young, single professionals who “have more of an affinity for the convenience, flexibility and affordability that renting brings.”

As someone who is personally invested in Arlington and wants to stay here for a while, I hope we learn from Seattle. That city didn’t have the opportunity to look at another place and see what was done right in partnership with Amazon and what could have been done better. We do.

This doesn’t have to be a horror movie. The script has not yet been written. That means there is still time to determine how this will end — happily ever after or with us haunted by this decision.